The Movement of West Nile Virus in North AmericaThe Movement of West Nile Virus in North America

UC BerkeleyUC Berkeley

October 1, 2003October 1, 2003

Lyle R. Petersen, M.D., M.P.H..Lyle R. Petersen, M.D., M.P.H..

Division of Vector-Borne Infectious DiseasesDivision of Vector-Borne Infectious Diseases

Fort Collins, ColoradoFort Collins, Colorado

Photo of Cx. quinquefasciatus courtesy of Dr. Stephen Higgs, UTMB/Galveston

Order of Topics

• What is it?• What is different now?• How did it get here?• How it is spreading?• How big is the problem?• What will happen next?• How do we prevent it?

West Nile Virus: BackgroundWest Nile Virus: Background

• RNA virus• Family Flaviviridae, Genus

Flavivirus• Member Japanese encephalitis

serocomplex• Closely related to SLE virus

• Isolation in 1937, West Nile district of Uganda

West Nile Virus:West Nile Virus:ApproximateApproximate Geographic Range Before Geographic Range Before

19991999

West Nile Virus Before and After Mid-1990s

• Before• Mild self-limited febrile illness• Central nervous system involvement rare• Periodic outbreaks

• After• Outbreaks associated with severe CNS illness• Some outbreaks associated with avian mortality• First recognition in North America (New York City)

in 1999

West Nile Outbreaks

• Israel 1941, 1951-1954, 1957, 1980, 2000

• France 1962, 2000• South Africa 1974• Romania 1996• Italy 1998• Russia 1999• United States 1999-2003

Arrival in North

America

Madagascar

Uganda 1937

Central African Republic2

Egypt 1951France 1962 South Africa 1974

NE United States

Israel 98/99/2000

India

Kunjin

Romania 1996Italy 1998Volgograd 1999

1

LINEAGEClade

Eg 101

U.S./Israel

Europe/Russia

India

Kunjin

Phylogenetic Tree of West Nile

Viruses

Israel 2000

An exotic arbovirus in NYC was surprising and unexpected:

Should it have been?

International Animal ImportationsInto New York

August 1998 – July 1999 N = 2,873,144

none1 – 60006001 - 2000020001 - 5000050001 - 350000350001 -

Animals

Top Ten Countries of OriginPassengers arriving into NY Airports

July 1998 – June 1999N = 4,850,090

Country Passengers PercentDOMINICAN REPUBLIC 539,740 11%UNITED KINGDOM 456,540 9%CANADA 350,510 7%FRANCE 312,400 6%MEXICO 310,330 6%ITALY 215,040 4%JAPAN 177,810 4%BRAZIL 157,220 3%ISRAEL 153,540 3%BAHAMAS 143,390 3%GERMANY 129,690 3%BERMUDA 114,750 2%

West Nile Virus Transmission CycleMosquito vector

Incidental infections

Bird reservoir hosts

Incidental infections

West NileVirus

West NileVirus

Incidental hostsHumansHorses

Other mammals

Other mosquito vectors*Culex salinariusCx.nigripalpus

Ochlerotatus sollicitansOc. taeniorhynchus

Aedes vexansAe albopictus

* With possible epidemic potential

?

?

Amplifying hostsPasserine birds

Enzootic/epizootic(amplifying) vectors Culex pipiens Cx. restuans Cx. quinquefasciatus Cx. tarsalis

West Nile Virus Amplification CycleWest Nile Virus Amplification Cycle

West Nile Virus Ecology in North America,as of Fall 2003

• 43 mosquito species• 170 native and captive avian species

• Mortality varies greatly by species• Very high viremias in some species• Bird-to-bird transmission documented

• Virus in feces and oral swabs• Oral infection possible

• 18 other species (e.g., cats, dogs, squirrels, chipmunks, harbor seal, alligator, bats, reindeer)

WNV Surveillance

• Dead birds (especially crows)• Human surveillance• Mosquitoes• Horse surveillance• Live captive sentinels (e.g. chickens)• Live wild birds• Zoo animals

•Reported to ArboNET as of 09/03/2003

Year # States # Counties Date Range

1999 4 28 9 AUG – 15 NOV

2000 12** 145 6 FEB – 17 NOV

2001 27** 359 8 APR – 26 DEC

2002 44** 2,531 3 JAN – 19 DEC

2003 45 1,954 1 JAN –29-SEP

States and Counties Reporting WNV Activity, United States, 1999-2003*

** Plus D.C.

Reported WNV Disease Cases in Humans,United States, 1999-2003*

* Reported as of 09/30/2003

Year # Cases # States # Counties Onset Date Range

1999 62 1 6 2 AUG – 24 SEP

2000 21 3 10 20 JUL – 27 SEP

2001 66 10 39 13 JUL – 7 DEC

2002 4,156 39** 740 19 MAY – 19 DEC

2003 5,722 41 730 28 MAR – 26 SEP

** Plus D.C.

Clinical EpidemiologyClinical Epidemiology

• Incubation period 2-14 days• May be longer in immunosuppressed

• Approximately 20% mild febrile illness• <1% severe neurological disease

• Meningitis, encephalitis, AFP• Advanced age primary risk factor for encephalitis• AFP patients may be younger• Immunosuppressive drugs and hematological

malignancies risk factors • Approximate 10% mortality• Long-term morbidity common

~80%Asymptomatic

~20%“West Nile Fever”

<1%CNS

disease

WNV Human Infection “Iceberg” in 2002WNV Human Infection “Iceberg” in 2002

284 fatalities

~ 3300 severe disease

~400,000

asymptomatic

~100,000

mild illness

0200400600800

1000120014001600180020002200

Jan

Feb

Mar

ch

Apr

il

May

June

July

Aug

Sep

t

Oct

Nov

Dec

Month of Onset

# ca

ses

0200400600800

1000120014001600180020002200

Jan

Feb

Mar

ch

Apr

il

May

June

July

Aug

Sep

t

Oct

Nov

Dec

Month of Onset

# ca

ses

Human WNV Illnesses (n=4156),Human WNV Illnesses (n=4156), by Month of Symptom Onset, 2002 by Month of Symptom Onset, 2002

1st Case

May 19

Last Case

Dec 19

0

10

20

30

40

50

60

Age group (yr)

Inci

denc

e pe

r M

illio

n*Human WNV Meningoencephalitis Incidence, Human WNV Meningoencephalitis Incidence,

by Age Group, United States, 2002by Age Group, United States, 2002

* States with human cases

0

5

10

15

20

25

30

35

40

Age group (yr)

Fata

lity

Rat

e (p

erce

nt)

Fatality Rate among Persons with WNV Fatality Rate among Persons with WNV Meningoencephalitis, by Age Group, United Meningoencephalitis, by Age Group, United

States, 2002States, 2002

0

10

20

30

Age group (yr)

Pro

po

rtio

n o

f d

isea

se c

ases

(%

)

WNME WNF

Human WNV Disease Cases, by Age Group and Clinical Category, United States, 2003*

* Reported as of 09/03/2003

Novel Modes of West Nile Virus Transmission, 2002

• Transplanted organs• One donor to four recipients

• Transfused blood• 23 confirmed cases in 2002, many more likely• WNV screening began in July

• Breast milk• One case, infant asymptomatic

• Transplacental transmission • One case, severe outcome to infant

• Percutaneous, occupational exposure

St. Louis Encephalitis

• Related Japanese encephalitis serocomplex flavivirus

• Endemic in the Americas• Similar maintenance vectors• Birds are main hosts• Causes human epidemics• Is SLE a model for the future behavior of WNV?

0

500

1000

1500

2000

2500

1932 1942 1952 1962 1972 1982 1992 2002

Reported and Estimated Number of St. Louis

Encephalitis Cases, U.S., 1932- Sept 12, 2003

0

500

1000

1500

2000

2500

1932 1942 1952 1962 1972 1982 1992 2002

Reported Number of West Nile Meningoencephalitis Cases, U.S., 1932- Sep 30, 2003

DCDC

SLE 1975

DCDC

WNME 2002*

>=10

1 – 9.9

0 – 0.9

Incidence per 106

Reported Incidence Reported Incidence in Humans:in Humans:SLE 1975SLE 1975

vs. vs. WNME 2002WNME 2002

* Reported as of 11/01/2002

West Nile Virus Future in the United States

• Worrisome for several reasons• Prolonged transmission season• Wide geographic range• Many potential amplifying avian hosts• High viremias in birds• Many potential mosquito vectors• Many potential human-biting mosquito species• Persistent infections in areas over 5 years• Urban / suburban / rural transmission

JOIN THE “SWAT TEAM” AGAINST

WEST NILE VIRUS

Prevention

• Repellants• DEET (up to 50% concentration); 10% in

children• Permethrin on clothing and fabrics

• Reduce mosquito breeding sites• Long sleeves and pants• Stay indoors when mosquitoes are biting (dawn

and dusk)• Bug zappers, sonic devices, CO2 devices

(mosquito magnet): no proven efficacy

Habitat elimination

Larviciding

Adulticiding